Provide Java 11 forward compatibility.

[blackwinter]

Prerequisite for #518.

[blackwinter]

Clarification: This pull request only allows the project to be built with Java 11, it doesn't require it. So it's a necessary prerequisite for #518, but it's not sufficient to "resolve" it.

This would be different for metafacture-fix where the upgrade of the Xtext Builder plugin (see metafacture/metafacture-fix#283) would make Java 11 a requirement to build the project.

(Also, it might be helpful to differentiate between build time and run time when talking about these requirements.)

[dr0i]

Don't we want to bump java version to 11? It seems easy - in build.gradle:

sourceCompatibility = 11
targetCompatibility = 11

Then change the path of java in your execution environment to java 11. Also set the environment variable JAVA_HOME to java 11.

Seems to work just out-of-the-box. Same with metafacture-fix, consuming the MF-core java 11 build.
What I've trouble with atm is to make it work with our play apps.

[blackwinter]

Sure, I'm all for it. That's just not what this pull request is doing.